1. Field of the Invention
This invention relates to a means for supplying an electric current to a helical antenna in a non-contacting manner, and more particularly to a two-wire or four-wire wound helical antenna.
2. Description of the Related Art
The conventional antenna devices of this kind include, for example, the 1/4 turn volute with split sheath balun of FIG. 6 for a "Resonant Quadrifilar Helix Antenna" reported on the Microwave Journal, December, 1970, p49-53, and the antenna device disclosed in Japanese Patent Laid-Open No. 30006/1988. FIG. 13 is a sketch drawing of the 1/4 turn volute with split sheath balun reported on the Microwave Journal. 101 denotes a first helical antenna radiation element pair, 102 a second helical antenna radiation element pair, 103 a coaxial feeder cable, 104 a 1/4 wavelength slit cut in an outer conductor of the coaxial cable 29, 105 an impedance conversion member provided on an inner conductor of the coaxial cable 103, and 106 a feeding point of the first and second helical antenna radiation element pairs 101, 102.
Both of the first and second helical antenna radiation element pairs 101, 102 can be regarded as balanced lines just as parallel two-wire lines in view of the operating condition thereof. Therefore, when an electric current is supplied to the antenna by connecting an unbalanced line, such as the coaxial cable 103 thereto, it is necessary to provide a balance-unbalance converter between the helical antenna radiation element pairs and coaxial cable. To meet this requirement, a balun comprising the coaxial cable 103, 1/4 wavelength slit 104 and impedance conversion member 105 is provided.
The conventional antenna device shown in FIG. 13 is formed as described above, in which the first and second helical antenna radiation element pairs 101, 102 are connected directly to the inner conductor of the coaxial feeder cable 103. Therefore, in order to move the helical antenna radiation element pairs 101, 102 as movable parts, the coaxial cable has to be moved simultaneously. This makes it difficult to move the helical antenna radiation element pairs, and, when the radiation element pairs are moved repeatedly, they are broken easily.
Since the antenna of a portable telephone has to be inserted and withdrawn easily, it is difficult to use the antenna of FIG. 13 for this purpose.